One of the more perplexing problems encountered by aircraft has been due to air disturbances such as clear air turbulence. Clear air turbulence is encountered at high altitudes and is thought to be due to at least two different conditions one of which is created by a standing wave found in the lee of a mountain barrier which occurs when statically stable air is carried over the mountain and the other of which results from waves formed in statically stable layers in the atmosphere that are subjected to sufficiently strong vertical wind gradients or shears.
It has heretofore been suggested that clear air turbulence can be detected by detecting temperature gradients existing therein. One such system utilizing detection of infrared or microwave energy from the CO.sub.2 band of the spectrum by use of a radiometric receiver is set forth by R. W. Astheimer in Applied Optics, Vol. 9, pg. 1789 (1970). In addition, a method and system for detecting clear air turbulence is set forth in U.S. Pat. No. 3,696,670. In this patent, it is suggested that detection be based on water vapor anomalies. Such detection is also the basis of the apparatus and method described in U.S. patent application Ser. No. 950,943, filed Oct. 13, 1978, by Peter M. Kuhn and entitled "Improved Method and Apparatus for Detecting Clear Air Turbulences", and now U.S. Pat. No. 4,266,130, issued May 5, 1981. Atmospheric absorption as a function of frequency and distance in infrared applications is discussed in "Infrared Passbands For Clear Air Turbulence Detection" by Kuhn, Nolt, Stearns and Radostitz in Applied Optics, Vol. 3, No. 4, October 1978.
While apparatus and methods have been heretofore suggested for detecting clear air turbulence, and improvements to such systems have also been heretofore suggested, further improvements are still deemed needed, particularly in providing a signal with a high signal to noise ratio to reduce the number of erroneous indications of clear air turbulences and/or in maintaining system calibration.